Particulates or particulate matter in exhaust gas from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbons and contains a trace of sulfate (misty sulfuric acid fraction). In order to reduce a discharged amount of particulates from the engine, a particulate filter is conventionally incorporated in an exhaust pipe through which the exhaust gas flows.
This kind of particulate filter is constituted by a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages; alternate ones of the passages have plugged inlets and the remaining passages with unplugged open inlets are plugged at their outlets. Thus, only the exhaust gas passing through thin porous walls compartmentalizing the respective passages is discharged downstream.
The particulates in the exhaust gas, which are captured and accumulated on inner surfaces of the thin porous walls, require to be burned off so as to regenerate the particulate filter before exhaust resistance considerably increases due to clogging. However, the exhaust gas from the diesel engine in a normal engine operation status rarely has a chance to obtain a temperature level at which the particulates ignite by themselves. Thus, used is a catalytic regenerative particulate filter integrally carrying an oxidation catalyst.
Use of such catalytic regenerative particulate filter will accelerate oxidation reaction of the captured particulates to lower their ignition temperature, so that the particulates can be burned off at exhaust temperature level lower than ever before.
However, even if such catalyst regenerative particulate filter is used, a captured amount may exceed a treated amount of particulates in engine operation areas with low exhaust temperature levels. Continued engine operation with such low exhaust temperature levels may hinder sufficient regeneration of the particulate filter, resulting in excessive accumulation of the captured particulates in the filter.
To this end, it has been conceived to additionally arrange a flow-through type oxidation catalyst in front of the particulate filter; with accumulation of the particulates becoming increased, fuel is added to the exhaust gas upstream of the oxidation catalyst to forcibly regenerate the particulate filter.
More specifically, fuel (HC) added upstream of the particulate filter undergoes oxidation reaction during its passage through the frontward oxidation catalyst. The exhaust gas heated by heat of the reaction and flowing into the particulate filter just behind increases a catalytic floor temperature of the filter to burn off the particulates, thereby regenerating the particulate filter.
This kind of fuel addition may be specifically realized such that fuel main injection at or near a compression upper dead center is followed by post injection at non-ignition timing after the compression upper dead center to add fuel into the exhaust gas.
Prior art references for such forced regeneration of particulate filter are, for example, the following Patent Literatures 1 and 2 by the same applicant as that of the present invention.                [Patent Literature 1] JP 2003-155915A        [Patent Literature 2] JP 2003-222040A        